This invention is generally related to micro-electromechanical system (MEMS) devices, and more specifically, to a variable capacitor that uses three-dimensional comb-drive electrodes which can be integrated into current state of the art semiconductor fabrication processes.
Variable capacitors or varactors play a fundamental role in high-frequency and radio-frequency (RF) circuits. In the last few years, MEMS variable capacitors have drawn considerable interest due to their superior electrical characteristics.
While variable capacitors using MEMS technology can be readily implemented in standard semiconductor devices for applications in aerospace, consumer electronics and communications systems, researchers have attempted to improve the tuning range of MEMS variable capacitors since the maximum capacitance tuning range achieved by parallel plate electrodes is limited. This is due to the non-linear electrostatic forces involved during actuation. Parallel plate electrodes exhibit a typical “pull-down behavior” at one-third the gap distance, leading to a maximum tuning capacitance of 1.5. Most previous approaches have resulted in an increased processing complexity and/or a large number of moving parts, leading to a drastic reduction in reliability. Additionally, packaging MEMS devices and integrating them into CMOS integrated circuits pose great challenges.
A. Dec et al., in an article entitled “RF micro-machined varactors with wide tuning range”, published in the IEEE RF IC Symposium Digest, pp. 309-312, June 1998, describe the construction of a MEMS variable capacitor by actuating a movable electrode using two parallel electrodes above and below the movable electrode. The total capacitance tuning range is significantly enhanced as a result of the individual capacitance between the movable-top and movable-bottom being in series. The maximum tuning range achievable using this approach is a ratio of 2:1. A. Dec et al. report achieving a tuning range as high as 1.9:1. Even though the tuning range significantly improves when using this approach, the process complexity increases correspondingly to a level that significantly reduces their utility for industrial applications.
U.S. Pat. No. 6,661,069 describes a method of fabricating a micro electro mechanical varactor using comb-drive electrodes as actuators. This approach is intended to increase the tuning range, but its construction, as described, involves fabricating the device on two separate substrates. The primary mode of actuation resides between the fin structures within the device. Further, the device is a three-port varactor and does not offer multiple actuating modes for enhancing the tuning range of the device.
In view of the foregoing considerations, there is a distinct need in industry for variable capacitors which construction differs considerable from the parallel plate devices and which method of fabrication differs from the conventional methods previously discussed. In particular, what is required are movable comb-drive electrodes for capacitance sensing and separate actuation electrodes for actuation of the movable comb drive electrodes. Preferably, the capacitance should vary by actuating one or more of the electrode fingers, thereby varying the overlap area between the comb electrodes. The capacitance tuning range of such device requires to be greatly enhanced by taking full advantage of multiple modes of actuation if possible in such devices. Since multi-port capacitors are required (i.e., at least two ports for DC bias and two ports for the RF signals), the signal capacitance should not require decoupling as is the case in conventional three-port varactors. The device should further be fabricated using standard semiconductor fabrication techniques and allow for an easy integrated into semiconductor circuits.
Accordingly, it is an object of the invention to provide a MEMS variable capacitor that utilized multi-fingered interdigitated three dimensional comb drive electrode for sensing, while the control or actuation electrodes drive the motion of the movable comb drive electrode beams either individually or all in unison, leading to changes in capacitance. It is another object to provide a MEMS varactor wherein the switch contacts are separated by a dielectric to provide electrical insulation between the ground electrode and the actuation electrode.
It is further an object to provide a MEMS variable capacitor with comb-drive electrode sensing for obtaining large capacitance ratio or tuning range.
It is yet another object to configure a plurality of MEMS variable capacitors in a variety of three-dimensional configurations.
It is still another object to provide a MEMS varactor having controlled stress gradient in the comb-drive electrode fingers leading to large change in overlap area.
It is still another object to provide a MEMS variable capacitor wherein the number and type of support structures to the movable comb drive fingers vary to lower the drive voltages.
It is still a further object to provide a method of fabricating a MEMS variable capacitor using manufacturing techniques that are compatible with applicable to CMOS semiconductor devices, which allows fabricating and packaging the MEMS device simultaneously and reduces the number of fabrication steps to a minimum while reducing the cost of processing.